<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      //用户输入一个数，  计算 1 到这个数的和。
      let num = +prompt('请输入一个数字')
      let sum = 0
      for (let i = 1; i <= num; i++) {
        sum += i
      }
      document.write(sum)
    </script>
  </body>
</html>
